/**
 * 获取 blob
 * @param  {String} url 目标文件地址
 * @return {Promise}
 */
function getBlob (url) {
  return new Promise(resolve => {
    const xhr = new XMLHttpRequest()

    xhr.open('get', url, true)
    xhr.responseType = 'blob'
    xhr.onload = () => {
      if (xhr.status === 200) {
        resolve(xhr.response)
      }
    }

    xhr.send()
  })
}

/**
 * 保存
 * @param  {Blob} blob
 * @param  {String} filename 想要保存的文件名称
 */
function saveAs (blob, filename) {
  if (window.navigator.msSaveOrOpenBlob) {
    navigator.msSaveBlob(blob, filename)
  } else {
    const link = document.createElement('a')
    const body = document.querySelector('body')

    link.href = window.URL.createObjectURL(blob)
    link.download = filename

    // fix Firefox
    link.style.display = 'none'
    body.appendChild(link)

    link.click()
    body.removeChild(link)

    window.URL.revokeObjectURL(link.href)
  }
}

/**
 * 下载
 * @param  {String} url 目标文件地址
 * @param  {String} filename 想要保存的文件名称
 */
export function download (url, filename) {
  getBlob(url).then(blob => {
    saveAs(blob, filename)
  })
}
